The present invention relates to a fixing device for use in an electrophotographic image forming apparatus, or similar devices.
Conventionally, a fixing method, which is widely used in an electrophotographic image forming apparatus, or the like, is a pressure-contact thermal-fixing method. In such a method, a recording medium (regular sheet, etc.), on which a toner image to be fixed is carried, is passed between a pair of rollers, at least one of which is heated and which are in pressure contact with each other during rotation, and then pressure-contacted and heated for fixing. In this case, thermoplastic resins, which are primary components for composing toner, are fused and softened, and adhered onto a recording medium, so that the toner image is fixed.
This method has the following excellent properties, in which fixing can be carried out at high speed, the overall size of the device can be reduced, and the possibility of fire is eliminated because the temperature of the thermal roller is only about 20.degree. C., which is a relatively low temperature. However, since toner and the thermo-fixing roller are in pressure contact with each other, some portion of toner adheres to the fixing rollers, that is, so-called offset phenomena tend to occur.
This problem also occurs in the fixing method, which is recently being used for lower speed devices, and in which a toner image is in pressure contact with a heat source through a heat resistive film.
Accordingly, various offset protection technologies are applied to the fixing device recently. Representatives of the above-described technologies are as follows. The surfaces of the fixing rollers are made of fluororesin which has low affinity to fused toner, and releasing agent such as silicone oil, etc., is coated onto the fixing roller.
On the other hand, recent toner improvement has occurred, and the control for molecular weight distribution of resin which is used for toner, and also the development of the releasing agents, contained in the toner, such as low molecular weight polypropylene, etc., have been advanced as disclosed in Japanese Patent Publication Open to Public Inspection Nos. 134652/1975 and 65231/1974.
However, even when these improved technologies are used, the surface release property of the recording medium from the fixing rollers is low during initial operations of the image forming apparatus, or when an image has excessive toner adhered amount on the leading edge of the recording medium, such as in the case of over-all black image, or the like. As a result, the following disadvantages can not be prevented. The recording medium is wound around the fixing roller, or toner adheres to the fixing roller, and the adhered toner is repeatedly transferred onto successive images, resulting in lowered image quality.
Further, requirements of the finished image quality has increased, and specifically, in color images and OHP images (over-head projector), it is required to increase the transparency of the finished image. Accordingly, it is necessary to use so-called sharp-melt resin, having a narrow rubber elastic area, as resin for toner. Therefore, the above-described requirements are disadvantageous for the toner-offset or sheet-winding.
Currently, there is no countermeasure except to increase the amount of silicone oil releasing agents, which is coated on the fixing roller. This countermeasure is insufficient for solving the problems for the following reasons. When the coating amount of the silicone oil is merely increased, the object to improve the releasing property is not always accomplished. Further, in order to liberally continue to coat silicone oil on the roller, it is necessary to provide a large sized fixing device having a large sized silicone oil tank. Still further, the following problems occur. Excessive oil adheres to and remains on the fixed recording medium, the finished image quality is lowered, and sticking occurs. Furthermore, conventionally, silicone rubber is used for the surface material of the fixing rollers. However, in this structure, the releasing property (separation property) of the recording medium from the fixing rollers, is low. Accordingly, oil, such as silicone oil, etc., is coated on the roller surface using an oil supply mechanism.
The present invention is accomplished as follows. In order to develop a fixing device in which the transparency of a formed image is high, the image quality is high, the size is smaller and the mechanism is simpler, there may be a method, in which low viscosity silicone oil (parting oil) is timely supplied in a predetermined amount onto the surface of the roller, without being localized on the surface of the roller.
However, it is difficult to hold low viscosity oil by a simple structure, and to ooze a predetermined amount of oil for a long period of time. For example, the following method, is provided, in which this structure is made non-porous to prevent excessive oozing, and as can be seen in the structure, made of Gore-rex, oozing is carried out by dispersion or penetration. Alternatively, there is another method in which a control layer is used and the diameter of holes provided in the layer is reduced. However, in the consideration of inventors of the present invention, it is difficult to control the oil coating amount, and to prevent oil from oozing at the time of non-use.
The object of the present invention is to solve the foregoing problems, and to provide a fixing device by which the high image quality is obtained on a finished image.
FIG. 7 is a view showing an example of the structure of the conventional device. In FIG. 7, numeral 101 is an upper fixing roller, and numeral 102 is a lower fixing roller. Numeral 103 is a recording sheet, used as a recording medium, which is sandwiched between these fixing rollers 101 and 102, and on which a toner image is fixed. Inside at least one of these rollers 101 or 102, a heating means for fusing the toner (not shown in the drawings) is provided. A toner image 103a is formed on the recording sheet 103. Numeral 104 is a guide for guiding the fixed recording sheet 103. Numerals 105 and 106 are sheet delivery rollers for delivering the recording sheet 103 sent through the guide 104.
Numeral 107 is an oil pad for coating, for example, silicone oil or the like, onto the surface of the upper fixing roller 101. Numeral 108 is an equalizing roller to level and equalize the oil coated on the roller surface by the oil pad 107. Numeral 109 is a web type (winding type) cleaning roller to remove toner adhered to the roller surface, which is composed of rollers 109a, 109b and 109c, and a ribbon-shaped cloth 109d. Numeral 110 is a fixing separation claw to separate the recording sheet 103 from the upper fixing roller 101 so that the fixed recording sheet 103 is not wound around the upper fixing roller 101 and is not carried therewith. Operations of the thus structured device will be explained below.
The toner image 103a, is developed by developing units (not shown) corresponding to an electrostatic latent image formed on a photoreceptor (not shown), is transferred onto the recording sheet 103, and the recording sheet 103 is conveyed to the fixing device by a conveyance mechanism. This toner image 103a may be a monochromatic toner image or a color toner image. In the fixing device, the upper fixing roller 101 and the lower fixing roller 102 are rotated in the direction as shown in the drawing.
While the recording sheet 103, sandwiched between the fixing rollers 101 and 102, passes between them, the toner image 103a is thermally fused, for example, at about 200.degree. C., and fixed onto the recording sheet 103. In these fixing operations of fixing rollers 101 and 102, for example, silicone oil is continuously supplied to the upper fixing roller 101 by the oil pad 107. Since the oil supplied from the oil pad 107 is usually mottled on the surface of the upper fixing roller 101, the equalizing roller 108 is rotated in the direction shown in the drawing so that the oil is uniformly adhered onto the roller surface.
While the recording sheet 103 passes between the upper fixing roller 101, onto which oil is adhered, and the lower fixing roller 102, since oil is adhered onto the surface of the upper fixing roller 101, it is difficult for toner to adhere onto the surface of upper fixing roller 101. However, since the toner is not completely removed from the roller surface, some toner remains adhering onto the surface of the upper fixing roller 101. Accordingly, any toner remaining on the roller surface is removed by the cleaning roller 109.
When the roller 109a is rotated as shown in the drawing and a web 109d is wound up, the ribbon-shaped cloth 109d is pushed onto the roller surface, and the toner is scraped off. By this operation, any remaining toner adhered onto the surface of the upper fixing roller 101 is removed.
In this connection, when the toner layer formed on the recording sheet 103 is thick, the thermal fusing force is strong and the recording sheet 103 is attracted onto the surface of the upper fixing roller 101. Accordingly, there is a possibility that the recording sheet 103 is carried with the upper fixing roller 101, without separating therefrom. This attraction of the recording sheet onto the roller surface frequently occurs when the solid toner layer is formed on the leading edge of the recording sheet. This phenomena cause jamming.
(Separability of the solid image)
In the case of monochromatic image fixing, because black toner is mainly used, elastic components are high and offset is barely occurs, when toner is fused in the nip portion (a portion which is pressed between the upper fixing roller 101 and the lower fixing roller 102 as indicated by 103c in FIG. 9). Even in this case, a document having solid image portions on the leading edge of the recording sheet (refer to FIG. 8) has a large toner amount, and excessively high thermal conductivity, resulting in offset. Accordingly, in order to prevent offset, oil is supplied by an oil supply pad, etc.
In the case of color image fixing, the color reproducibility and glossiness of a transparency sheet are necessary, and it is necessary to form a smooth surface for the toner image after fixing. Therefore, when toner is fused in the nip area, toner viscosity is greatly lowered, and the toner tends to be adhered to the roller. Accordingly, more offset resistivity is required than in monochromatic image fixing. Therefore, a large amount of oil is coated onto the roller. In this case, in order to match the monochromatic image fixing, silicone rubber, which has high affinity to oil, is used for the material of the upper fixing roller. In this case, since the roller material is rubber, durability is limited due to oil swelling, etc.
In order to more easily separate the recording sheet from the roller, a fixing separation claw 110 is brought into contact with the surface of the upper fixing roller 101, and the recording medium 103 is assuredly separated.
In the above example, a fixing device utilizing fixing rollers has been explained. However, there exists a fixing device in which a film sheet is used for fixing. FIG. 10 is a view showing an example of another structure of the conventional fixing device. A low thermal capacity line heater 125, which is fixed to and supported by the device, is made of an alumina base plate 127 on which resistance material is coated, and a current is impressed from both ends. The current flow has a pulse wave-form, and temperature is detected by a temperature sensor 126. The pulse width is changed so that the temperature becomes a predetermined value.
Numeral 124 is a film sheet onto which a toner image, formed on the recording sheet, is fixed. The film sheet 124 is conveyed without wrinkles and slippage due to the drive and tension by a film drive roller 121 and a film driven roller 122. For the film sheet 124, a 10-35 .mu.m heat-resistive film is used. For the material of the film sheet 124, a film is used on which at least 5-15 .mu.m releasing agent layer, which is made by adding a conductive material to fluororesin such as teflon, etc., is coated on, for example, polyester.
A pressure roller 123 with a rubber elastic layer, such as silicone rubber, which has a high releasing property, is operated as follows. The pressure roller 123 presses the recording sheet (not shown) onto the line heater 125 through the film sheet 124, and is rotated in the arrowed direction. The recording sheet, on which the toner image is formed, is passed between the film sheet 124 and the pressure roller 123, is passed through the line heater 125, and is fixed. The fixed recording sheet is separated from the fixing mechanism by the fixing separation claw 128, and is guided to a sheet delivery mechanism, which is not shown.
Also in this film sheet type fixing apparatus, the same oil coating mechanism and the remaining toner removal mechanism, as these shown in FIG. 7, are provided, so that the separability of the recording sheet from the film sheet 124 is improved, and any toner adhered onto the film sheet 124 is removed.
In the above-described conventional fixing apparatus, an equalizing roller is used so that oil adheres uniformly onto the surfaces of the roller and film sheet. Further, in order to remove any remaining toner adhered onto the surface of the fixing roller or the surface of the film sheet, the cleaning roller is used. Accordingly, the structure of the device has become excessively complicated, and the cost of the device is increased. Further, since the recording medium separation claw 110 or 128 is brought into pressure-contact with the surface of the roller or the surface of the film sheet, the edge of the separation claw is worn. Accordingly, the separation claw which loses its separation ability, requires replacement, and causes additional maintenance, which is disadvantageous.